Retaining Wall Calculator

This is an estimate, not professional advice. Check your inputs and verify the result against your plans and local building code before you build or order. See terms and disclaimer.

How this calculator works

Wall area (ft²) = Length(ft) × Height(ft)Blocks per ft² = 144 ÷ (Block length(in) × Block height(in))Blocks = Area × blocks per ft² × (1 + Waste% ÷ 100)Courses = Height(in) ÷ block height Cap row = blocks per courseBase pad (ft³) = (2 × block depth ÷ 12) × (6 in ÷ 12) × Length yd³ = ÷ 27Drainage gravel (ft³) = Wall face area × 1 ft × 1.1 yd³ = ÷ 27

Enter your dimensions and the result updates instantly. A waste allowance is included so you order slightly over rather than running short mid-pour, and ready-mix is rounded up to the nearest quarter yard, which is how it is sold.

Worked example

A 20 ft long by 3 ft high wall with 12 by 8 inch face blocks and a 10% waste allowance. Inputs: Wall length 20 ft, Wall height 3 ft, Block face length 12 in, Block face height 8 in, Waste allowance 10 %. Result: 99 .

Blocks for common retaining walls

Calculated for a 12 by 8 inch block face with a 10% waste allowance. Tap a size to load it above.

Slab size	Wall blocks	Courses (rows high)
20 ft × 2 ft	66	3	Use →
20 ft × 3 ft	99	5	Use →
30 ft × 3 ft	149	5	Use →
40 ft × 3 ft	198	5	Use →
30 ft × 4 ft	198	6	Use →
50 ft × 4 ft	330	6	Use →

Method & assumptions

A segmental retaining wall is built from interlocking blocks, so the count is the wall face area divided by the face area of one block. We take 144 square inches per square foot and divide by the block face (length times height) to get blocks per square foot, multiply by your wall area, add waste, and round up.

Courses are the wall height in inches divided by the block height, and blocks per course is the wall length divided by the block length. The top course is usually finished with cap blocks, which we count separately as one row. Because face sizes vary a lot between systems, enter the actual dimensions from your block, not a generic guess.

Block count is only part of a retaining wall, so this calculator also sizes the two gravel layers. Below the first course you need a compacted leveling pad, estimated here about 6 inches deep and twice the block depth wide along the wall. Behind the wall you need a drainage layer, estimated as a 12 inch zone of clean gravel over the wall face with 10% added for voids, with a pipe at the base so water pressure does not push the wall over. Walls over about 3 to 4 feet, or any wall holding a slope or surcharge, usually need engineering and geogrid reinforcement, so check local code before you build.

Pro tips and common mistakes

Start with a buried base course. Set the first course on 6 inches of compacted gravel and bury it about one tenth of the wall height. A dead-level, well-compacted base is what keeps the wall straight for years.
Drain behind the wall. Backfill with clean gravel and run a perforated drain pipe at the base. Trapped water is the number one reason block walls bulge and fail.
Respect the height limit. Above roughly 3 to 4 feet, or with a slope or driveway above, walls need an engineer and geogrid reinforcement. Do not freehand a tall wall.
Use the block setback. Most systems lean the wall slightly into the hill as it rises. Keep the built-in setback consistent so the wall resists the soil instead of fighting it.
Order caps and corners. Cap blocks and corner units are separate from field blocks and sometimes special order. Count them early so the top course is not held up.

How we check this

Every formula is shown in full and verified with automated tests against manufacturer spec sheets and ASTM and ACI standards. We show our work so you can check it.

Frequently asked questions

How many blocks do I need for a retaining wall?

Divide the wall face area by the face area of one block. For a 12 by 8 inch block (1.5 blocks per square foot), a 20 by 3 foot wall (60 square feet) needs about 99 blocks with 10% waste, plus a cap row.

How high can a retaining wall be without engineering?

Many codes allow segmental walls up to about 3 to 4 feet without an engineer, but a slope, driveway or other load above the wall lowers that limit. Always check local code before you build.

What goes behind a retaining wall?

A drainage layer of clean gravel with a perforated pipe at the base, so groundwater drains instead of building pressure against the wall. This is essential, not optional.

Do I need cap blocks?

Cap blocks finish the top course and are usually glued down. They are counted separately here as one row, and are often a different unit from the field blocks.

References

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